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Microbiology and Diet

K. Dhunjibhoy

The microflora of the gut varies according to the milieu intereur of the body. Various factors influence the microflora of the oral cavity and the gut.

Let us first consider the normal microflora and the time in growth when the organisms start living in the host.

Mouth: The bacteria present in the mouth are subject to great variation both in number and in kind. Even the fairly clean and healthy mouth contains a considerable amount of detritus and other organic matter derived from particles of food, desquamated epithelium, pharyngeal mucus and other sources; these provide nutrition to the flora which frequently change.

The saliva as is initially secreted, is sterile but contains growth promoting substances for members of a few species such as lactobacilli, Shigella, salmonella, bacillus, vibrio. The following organisms are found in the mouth and saliva:

  1. Micrococci are the commonest—They can be pigrnented / non pigmented, aerobic / anaerobic.
  2. Staph. albus a common inhabitant, but staph. aureus also commonly inhabits the mouth.
  3. Streptococci—haemolytic streptococci are constantly present; haemolytic It streptococci are seen in 5%-10% of healthy throats.
  4. Gram positive bacilli in chains—they belong to the Lactobacilli group (some to the group of aerobic spore-bearers.) Lactobacilli are abundant in cases of dental caries.
  5. Gram negative bacilli including members of the coliform and proteus groups.
  6. Spirochaetes, which are almost invariably present between the gums and the teeth are Treponems buccalis, dentium, intermedium, microdentium, macrodentium, Vincent's Spirillum and Miller's spirillum.
  7. Actinomyces spp.
  8. Fusobacterium spp.
  9. Pleuropneumoniae-like organisms.
  10. Yeasts—mainly Candida albicans.
  11. Nocardia spp.
  12. Neisseriae spp.
  13. Corynebacterium spp
  14. Leptotrichia spp.

Counts of organisms per ml of saliva are

  • 2.5 x 107 lactobacilli
  • 1.2 x 107 proteolytic organisms
  • 4 x I06 veillonella and fusobacterium spp. each
  • 6.3 x 103 Staphylococci
  • 30—Proteus

In infants the mouth becomes colonized shortly after birth; staphylococci, streptococci, lactobacilli and coliform bacteria being readily detectable. Within a few days the flora is largely that of the adult. The maternal vagina and later the upper respiratory tract appear to be the source of the colonizing organisms. The intestine, at birth contains at most a few bacteria, is colonized rapidly "per os" and to some extent "per anum".

The intestinal flora of the breast-fed infant consists largely of anaerobic lactobacilli of the Bifidobacterium group. They may constitute 99% of total viable organisms in the faeces. Secliger and Werner record counts of 1010 per gm of stool as compared to 109 in adults and a predominance of 3 to 300 fold over the remainder of the flora. Coliforms, enterococci, staphylococci and aerobic lactobacilli are also present.

With weaning, or in bottle-fed infants the flora tends to resemble that of the adult, the number of anaerobic lactobacilli declines slightly; bacteria of the fusobacterium group and proteolyiic bacilli appear in large numbers; and small numbers of aerobic and anaerobic spore-bearing organisms appear.

In adults the empty stomach is generally sterile. Immediately after a meal it contains numerous organisms which have been ingested with food, but these with the exception of acid resistant vegetative bacilli and sporing bacteria, appear to be killed rapidly. If however the motility of the stomach is excessive or the acidity is less than normal, this sterilizing effect of gastric juice is incomplete. Thus, in cases of gastric disease particularly carcinoma, saranae, saprophytic bacilli and other organisms may multiply in the stomach.

In the healthy adult the jejunum and upper ileum are practically sterile.

The number of organisms mainly facultatively anaerobic Streptococci viridans, staphylococci, lactobacilli and fungi increase from the stomach to the ileocaecal valve beyond which the flora becomes much more abundant and qualitatively deficient. The duodenum may contain 100-1000 organisms per ml, the jejunum 1000-10,000, the upper ileum about 1,00,000 and the lower ileum 1,000,000, but there is a lot of variation from sample to sample.

Coliform bacilli appear in the jejunum in certain diseases such as infantile gastroenteritis, hepatic cirrhosis after gastrectomy and in chronic debilitated states.

The flora of the lower part of the ileum is relatively scanty but includes most of the organisms found in abundance in the large intestine. The flora of the large intestine and faeces is same.

Anaerobic bacteria make up about 99% of the faecal flora. Most of the anaerobic bacteria belong to Gram-positive bifidobacteria and Gram-negative bacteroides (their number varies from 109 to 101 per gm of faeces). Lactobacilli, clostridia and fusobacterium, average each about 103 to 105 per gram; Enterobacteria 106 and enterococci 106 per gram. Less frequent are proteus spp., pseudomonas aeruginosa and spirochaetes.

Among the entero bacteria E. coli is the commonest. Klebsiella and other enterobacters follow.

Importance of Micro Organisms in the Intestine

Presence of bacteria in the intestine is very essential for the life of the host for the following reasons. They assist in the digestive processes and without them much of the food taken in would be passed out of the body in an unassimilable condition.

Production of vitamin K, biotin, folic acid and riboflavin by the gut flora is very well known. Also the gut flora prevents other intestinal pathogens from invading the mucosa.

The factors which influence the intestinal flora are

  1. Host-Physiology
    Intestinal secretion
    Intestinal mucosa
    Immune mechanisms
  2. Environmental factors
    Bacterial contamination
    Antibacterial drugs
  3. Bacterial Interaction

The faecal flora is a lot dependent on the diet and is different in different countries. For example bacteroides spp. are found in great numbers in subjects who consume mixed western diet that contains a large amount of fat and stimulates the production of a large volume of bile. In people consuming vegetarian diet, fewer bacteroides spp. are found. In people from Africa, Asia and Japan, who consume a vegetarian diet Gram positive anaerobes and enterococci are the predominant faecal organisms. However subjects who change to a vegetarian diet do not convert their faecal flora to a Gram positive predominance but retain their bacteroides organism.

The following relationship between diet and the organisms isolated from the faeces has been found.

Diet Mixed Western Wholly or Primarily Veg.
Country of Origin USA Scotland England Uganda Rice
Enterobacteria 7.4 7.6 7.9 7.4 7.9 8.0 9.4 7.0
Enterococci 5.9 5.3 5.8 5.3 7.3 7.0 8.1 4.8
Lactobacilli 6.5 7.7 6.5 5.3 7.6 7.2 7.4 7.4
Clostridia 5.4 5.6 5.7 4.7 5.7 5.1 5.6 5.4
Bacteroides 9.7 9.8 9.8 9.8 9.2 8.2 9.4 9.7
Gram Positive sporing anaerobes 10.0 9.9 9.8 9.5 9.6 9.3 9.7 9.6

subject living on strictly veg. diet.
(Figures indicate loglO number of bacteria per gram of faeces.
Western—Diet—rich in fat and animal proteins)
(Matoke—boiled mashed banana)

Tbe people living on the high carbobydrate diet had significantly fewer bacteroides and more enterococci in their faeces than did those on a Western diet; aerobic bacteria were also more prominent.

Draser B.S. (1974) corroborated the previous study. His findings of faecal flora in different human populations are tabulated below:

Faecal Flora of Different Human Population

Diet Country Mean Log10 number of Bacteria/gm of Faeces
Clostridia Bacter-
India 7.9 7.3 7.6 5.7 9.2 9.6 9.5
Japan 9.4 8.1 7.4 5.6 9.4 9.7 9.6
Uganda 80 7.0 7.2 5.1 8.2 9.4 9.3
England 7.9 5.8 6.5 5.7 9.8 9.9 9.3
Scotland 7.6 5.3 7.7 5.6 9.8 9.9 9.3
U.S.A. 7.4 5.9 6.5 5.4 9.7 9.9 9.3


Bacteroids occurred in greater number in (English, Scots & Americans) developed counties.
Eubacteria was prominent in India & Japan.
Enterococci-most numerous in India, Japan and Uganda.
Streptococcus faecalis-dominant in English, Scots and Arnericans.
Streptococcus faecium—dominant in India, Japan and Uganda
Cl. perfringens & bifermentans—occurred in all groups.
Cl. Paraputrificum—English. Scots & Americans but rare in other sources.

Recently sarcina ventriculi, type of organism, has been isolated in the stools of vegetarian people. The.importance of this will be evident very soon.

Thus it may be noted that since the bacterial flora appears to be strongly dependent on the kind of food we eat, it is obvious that we must select the food we eat very carefully. The bacterial population in our intestines is ultimately going to enhance .the value of the food by improving the efficiency of digestion and also by the manufacture of important vitamins as mentioned above. All this can occur only if the bacteria are congenial and not if they are pathogenic or non-contributory to the good health of the host.


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